Pressure-sensitive adhesive tapes such as packing tapes and surface-protective tapes and pressure-sensitive adhesive labels, seals, sheets, etc. for industrial and office use are produced by applying a pressure-sensitive adhesive composition mainly comprising an elastomer and a tackifier resin onto a backing support such as kraft paper, polyester cloth and polyethylene film.
In recent years, use of a hot-melt and solvent-free pressure-sensitive adhesive composition has been recommended from the viewpoint of regulations for environmental protection, energy-saving, resources-saving, etc. In this connection, a styrene-isoprene-styrene block copolymer has been widely employed as the elastomer for use in such a pressure-sensitive adhesive composition.
Pressure-sensitive adhesive compositions are required to have excellent pressure-sensitive adhesive properties such as tackiness, peel strength and holding power. Particularly, demand exists for a pressure-sensitive adhesive composition which is excellent in the tackiness at low temperatures, the peel strength at ordinary temperatures and the holding power at high temperatures. Extensive studies have been made to enhance these pressure-sensitive adhesive properties.
For example, Japanese Patent Application Laid-Open Nos. 63-178187 and 63-178188 disclose a pressure-sensitive adhesive composition containing a styrene-isoprene-styrene block copolymer and a styrene-isoprene diblock copolymer. The disclosed pressure-sensitive adhesive composition exhibits a relatively high peel strength at ordinary temperatures, but is unsatisfactory in the tackiness at low temperature or the holding power at high temperatures.
The production of a pressure-sensitive adhesive label generally includes a step of die-cutting a pressure-sensitive adhesive sheet adhered to a releasing sheet into the shape of intended pressure-sensitive adhesive label. When a pressure-sensitive adhesive composition having poor die-cuttability is employed in this step, the cutting of the pressure-sensitive adhesive sheet cannot be attained successfully. Thus, the resultant pressure-sensitive adhesive labels become stuck to one another, thereby considerably reducing the product value, and in addition, the pressure-sensitive adhesive composition adheres to the die, thereby adversely affecting the subsequent cutting operations. To solve the above problems, various studies have been made to improve the die-cuttability.
For example, U.S. Pat. No. 5,290,842 proposes a pressure-sensitive adhesive composition containing a styrene-isoprene-styrene block copolymer and a styrene-butadiene-styrene block copolymer. Japanese Patent Application Laid-Open No. 2001-504519 discloses a pressure-sensitive adhesive composition comprising a styrene-isoprene-styrene block copolymer, a styrene-isoprene diblock copolymer and a tackifier resin, which exhibits a single glass transition temperature. However, the proposed pressure-sensitive adhesive compositions are insufficient in the improvement of die-cuttability or reduced in the pressure-sensitive adhesive properties, thereby failing to be excellent in both the pressure-sensitive adhesive properties and the die-cuttability.